This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.
Modern drug discovery frequently identifies active pharmaceutical ingredients (APIs) that are highly effective against the disease target, but which are hard to deliver to the body. Increasingly, new compounds have suboptimal aqueous solubility, whereby it is estimated that some 40% of new chemical entities are discarded for this reason. Solubilization strategies are therefore essential. One of the most common methods towards this end is the creation of amorphous formulations, which requires kinetically trapping the API in a non-crystalline metastable form to increase the dissolution rate and transient concentration. Unfortunately, such formulations run the risk of spontaneously transitioning to their more thermodynamically stable crystalline form. If such phase changes occur, significant reductions in drug bioavailability may arise.
For intravenous (IV) formulations in particular, it is essential to be able to either completely dissolve the drug in a physiologically compatible vehicle, or to administer a suspension with particles small enough so that they will not occlude blood vessels. Therefore, various different strategies to solubilize APIs are employed, including pH adjustment, the use of cosolvents, surfactants, and emulsifiers, and most recently nanosuspensions. Unfortunately, additives used to solubilize drugs for IV formulations can cause toxicity, in some cases fatal. Therefore, solubilization strategies that minimize the use of potentially toxic additives are of great interest. One interesting example of a poorly water soluble drug that was reformulated to avoid toxicity issues and to improve efficacy is paclitaxel, a vital anticancer drug with activity against several human cancers. The original formulation contained the drug solubilized in ethanol and polyethoxylated castor oil, however, the solubilizing vehicle contributed to serious hypersensitivity reactions. An alternative formulation was subsequently developed whereby paclitaxel was coprecipitated with human serum albumin leading to an amorphous formulation. Upon reconstitution, this formulation is a nanosuspension containing particles of 120 nm.
Quantification and detection of crystallinity at low levels within an amorphous formulation is often a defining measurement for predicting the long-term success of the formulation. Crystal formation is detrimental because it not only reduces the solubility advantage of the amorphous formulation, but the presence of large particulates in IV formulations is problematic and hence subject to stringent regulations. Unfortunately for potent APIs at low loadings, accurate determination of crystallinity poses a significant measurement challenge. As the drug loading approaches the detection limits of conventional bench top methods, even major differences in relative drug crystallinity become difficult to distinguish with statistical confidence. Commonly used methods for crystal detection in the drug formulation pipeline include X-ray powder diffraction (PXRD), differential scanning calorimetry (DSC), Raman spectroscopy, scanning electron microscopy, hot stage microscopy, and nuclear magnetic resonance spectroscopy (NMR). However, none of these techniques typically exhibit detection limits for crystallinity significantly lower than ˜1%, in most cases because of background noise from the much larger non-crystalline fraction.
Nonlinear optical (NLO) microscopy, in particular second harmonic generation (SHG), has recently emerged as a complementary technique for the rapid detection and quantification of trace crystallinity within pharmaceutical materials. Coherent SHG selectively arises exclusively from the bulk crystalline fraction and only within crystals of appropriate symmetry. The chirality inherent in most new pharmaceutical APIs largely guarantees that the crystals produced will be bulk-allowed for SHG. This selectivity for crystalline API has allowed measurements with detection limits on the order of parts per billion to as low as parts per trillion ranges under favorable conditions.
Despite the low detection limits of SHG microscopy, the SHG intensity itself provides little significant chemical information about the composition of the SHG-active source. In the present disclosure, efforts were undertaken to lower the detection limits of both Raman and XRD through background suppression, guided by SHG imaging. In brief, targeting Raman and XRD analysis to regions of interest identified by SHG minimizes the volume of additional material contributing to the signal and greatly reduces the background from the amorphous material.
There is therefore an unmet need to lower the detection limits of both Raman and XRD.